1. Field of the Invention
The present invention relates to a drive circuit for voltage-controlled transistors, and more particularly to an improved drive circuit that reduces the voltage surge applied to an inverter circuit during the occurrence of overcurrent.
2. Description of Related Art
A great number of power switching circuits using voltage-controlled transistors have been proposed. There are various kinds of voltage-controlled transistors. For example, there are insulated gate bipolar transistors (hereinafter referred to as IGBTs) which have an insulated gate and operate in the bipolar mode, insulated gate field-effect transistors which have an insulated gate and operate in the field-effect mode, and metal-oxide-semiconductor field-effect transistors.
A chopper circuit using a general gate drive circuit is shown in FIG. 9 and this gate drive circuit is described in IGBT DESIGNER'S MANUAL (E-17, 1994), published by International Rectifier.
In FIG. 9, a diode 2 and a load device 3, connected in parallel, are connected to a dc voltage source 1 in series. The collector (C) of an IGBT 5 is connected to the parallel circuit comprising of the diode 2 and the load device 3. An inductor 4 exists between the IGBT 5 and the parallel circuit. The IGBT 5 has an emitter (E) which is connected to the negative side of the dc voltage source 1 and a gate (G) which is connected to a drive circuit 6 described below. If a positive voltage is applied between the gate (G) and the emitter (E), current will flow between the collector (C) and the emitter (E). Alternatively, if the voltage between the gate (G) and the emitter (E) becomes less than a threshold voltage, current won't flow between the collector (C) and the emitter (E).
IGBT includes C.sub.GC 51, the collector-to-gate capacitance, and C.sub.GE 52, the gate-to-emitter capacitance of the. A drive circuit 6 turns the IGBT 5 on and off. The drive circuit 6 is equipped with a dc voltage source 61 for driving the drive circuit 6, a switch 62 which is turned on to apply a positive voltage to the gate of the IGBT 5 when turning on the IGBT 5, a switch 63 which is turned on to make the gate of the IGBT 5 a voltage of zero volts when turning off the IGBT 5, and a resistor (gate resistor) 64a which determines the charging-discharging constant of the gate of the IGBT 5. A signal generating circuit 7 for generating "on" and "off" commands is connected to the switches 62 and 63 of the IGBT 5. The behavior and operation of the aforementioned conventional circuit of the present invention will next be described.
If the switch 63 is turned off and the switch 62 is turned on by the the signal generating circuit 7, a positive voltage is applied to the gate of the IGBT 5 through the resistor 64a, so the IGBT 5 is turned on and a current flows from the dc voltage source 1 through the load 3. Next, if the switch 62 is turned off and the switch 63 is turned on, the gate voltage of the IGBT 5 becomes less than a threshold voltage, so the IGBT 5 is turned off and the current flowing through the load 3 returns through the diode 2.
Suppose that the load 3 is shorted when the IGBT 5 is on, various waveforms of the chopping circuit in FIG. 9 are shown in FIG. 10. In FIG. 10, the voltage on the connection point between the switches 62 and 62 is represented by V.sub.s, the gate-to-emitter voltage of the IGBT 5 by V.sub.GE, and the collector-to-emitter voltage of the IGBT 5 by V.sub.CE, and the collector current of the IGBT 5 by I.sub.C.
Assuming that at time T0 the load 3 is shorted, the collector current I.sub.C will abruptly rise, the IGBT 5 reaches the operation of the active region, and V.sub.CE rises. As V.sub.CE rises, a current expressed by the following equation flows from the collector of the IGBT 5 into the gate of the IGBT 5 and the gate voltage rises (time T1 to time T2).
C.sub.GC .times.dV.sub.CE /dt PA1 C.sub.GC .times.dV.sub.CE /dt
In general, a resistor R.sub.g is inserted into the gate of the IGBT for smoothly switching the IGBT on and off. Suppose the voltage of the drive-circuit voltage source 61 is Vg1. When an "on" command is given from the signal generating circuit to the IGBT 5, the gate voltage V.sub.GE of the IGBT 5 rises to a value shown by the following equation, and because of the device characteristic of the IGBT 5, an even larger current flows. EQU V.sub.GE =Vg1+C.sub.GC .times.dV.sub.CE /dt.times.R.sub.g
Overcurrent, although not shown, is usually detected and the IGBT 5 is turned off so that it is not destroyed by overcurrent. Because the current of the IGBT 5 changes at the turn-off time, the following voltage is applied between the collector and the emitter of the IGBT 5 (time T2 to time T3). EQU V.sub.CE =V.sub.DC +L.times.di/dt
This voltage is the surge voltage (which is expressed by the product of the inductance L of the inductor 4 and the rate of change of the collector current I.sub.C of the IGBT 5) superimposed on the magnitude V.sub.DC of the dc voltage source 1. The rate of change of the collector current at this time is determined by the device characteristic of the IGBT 5 and is approximately proportional to the rate of change of the gate voltage of the IGBT 5. Therefore, if the rate of change of the gate voltage becomes large, the rate of change of the collector current will also become large. If an "off" command is given to the IGBT 5 when the gate voltage of the IGBT 5 is rising, the rate of change of the gate voltage is large and thus the rate of change of the collector current also becomes large. As a result, the collector-to-emitter voltage becomes large and, in the worst case, the IGBT 5 is sometimes damaged.
Another prior art publication will next be described.
A diode 65 is provided in Japanese Patent Laid-Open No. 63-95728 for suppressing a rise in the gate voltage of the IGBT, as shown in FIG. 11. Others are the same as the aforementioned prior art shown in FIG. 10 and so a description will not be given.
The behavior and operation of the prior art shown in FIG. 11 will hereinafter be described.
Assuming that at time T0 the load 3 is shorted, the collector current I.sub.C will abruptly rise, the IGBT reaches the operation of the active region, and the collector current V.sub.CE rises (time T1 to time T2). As the collector current rises, a current expressed by the following equation flows from the collector of the IGBT through the collector-to-gate capacitance C.sub.GC into the gate of the IGBT, and the gate voltage rises.
In general, a resistor R.sub.g is inserted into the gate of the IGBT for smoothly switching the IGBT on and off. Assume the voltage of the voltage source of the gate circuit is Vg1. When an "off" command signal is output from the signal generating circuit 7 to the IGBT 5, the gate voltage V.sub.GE of the IGBT tries to reach a value indicated by the following equation, but the rise in the gate voltage is limited to the value of the following equation due to the operation of the diode 65 (time T2 to time T3). EQU V.sub.GE =Vg1+C.sub.GC .times.dV.sub.CE /dt.times.R.sub.g
If between time T2 and time T3 the rise in the gate voltage is suppressed due to the operation of the diode 65, the diode is turned on by the time the collector current I.sub.C begins to fall, after the operation delay time .DELTA.T of the IGBT 5. Therefore, as if the IGBT is turned on when the resistance of the gate resistor is zero, the collector current decreases abruptly at a rate of dI.sub.C /dt, and the voltage represented by the following equation occurs until the collector current reaches its stable state. As a result, in the worst case the IGBT is sometimes damaged . EQU V.sub.CE =V.sub.DC +L.times.dI.sub.C /dt
Thereafter, an overcurrent detecting circuit, although not shown, usually detects overcurrent, and the IGBT 5 is operated so that it is turned off (after T4).
The conventional drive circuits for a voltage-controlled transistor such as an IGBT, as described above, have the problem that a large voltage surge occurs and in the worst case an IGBT is sometimes damaged, when a large current due to circuit failure flows and the IGBT operates in the active region, or when current is cut off during the operation in the active region.